[Leejohnston]

## Entropy — Why Things Fall Apart

### 1. The Equation

Change in entropy:

ΔS = Q ÷ T

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### 2. What Each Symbol Means

- ΔS = change in entropy
- Q = heat added
- T = temperature (kelvin)

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### 3. What the Equation Is Telling Us

Entropy measures **energy spreading and disorder**.

Higher entropy states are more likely.

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### 4. Where It Comes From (Intuition)

Energy naturally spreads out.

Ordered systems tend to become disordered unless energy is added.

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### 5. Real-World Examples

- Ice melting
- Heat flowing from hot to cold
- Mixing gases

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### 6. Why Time Has a Direction

Entropy increasing gives time its arrow.

Physics laws work both ways — entropy doesn’t.

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### 7. Common Misconceptions

- Entropy means chaos → incomplete
- Entropy always increases everywhere → false (locally)
- Entropy is about mess → misleading

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### Try It Yourself

Why doesn’t heat flow from cold to hot by itself?